1. Field of the Invention
The present invention relates to a process for producing .alpha.-L-aspartyl-L-phenylalanine methyl ester (hereinafter abbreviated as .alpha.-APM), which is about 200 times sweeter than sucrose and is extensively required as a sweetener due to its good sweet taste and low calorie content. More particularly, the present invention relates to a process for recovering L-phenylalanine (hereinafter abbreviated as L-Phe) from an aqueous solution such as a side flow (effluent) generated from a process for producing .alpha.-APM.
2. Discussion of the Background
Various processes for producing .alpha.-APM are known including, for example, a process which involves the reaction between an N-protected L-aspartic anhydride and L-phenylalanine methyl ester (U.S. Pat. No. 3,786,039), a process which involves the reaction between an N-protected L-aspartic anhydride and L-Phe (U.S. Pat. No. 4,173,562), and a process which involves the enzymatic condensation of N-benzyloxycarbonyl-L-aspartic acid and L-phenylalanine methyl ester (Japanese Patent Application Laid-Open No. 92729/1978).
However, in order for such processes to be useful as low-cost industrial processes, it is required to recover and reuse the L-Phe and L-aspartic acid (hereinafter abbreviated as L-Asp) contained in side flows (effluents) obtained from the production of .alpha.-APM, for example, the mother liquor from the crystallization of .alpha.-APM or the mother liquor from the crystallization of .alpha.-APM HCl since these side flows contain a significant amount of L-phenylalanine derivatives, L-aspartic acid derivatives, .beta.-L-aspartyl-L-phenylalanine methyl ester and .alpha.-APM. Particularly, it is very important to efficiently recover L-Phe, which is an expensive amino acid.
A process for recovering L-Phe from the side flow generated from a process for the production of .alpha.-APM is disclosed in Japanese Patent Application Laid-Open No. 7812/1973. This process involves hydrolyzing a solution containing .beta.-L-aspartyl-L-phenylalanine methyl ester in the presence of a mineral acid, adjusting the pH to about the isoelectric point of L-Phe, i.e., about 6, by adding aqueous alkali to the reaction solution to crystallize and separate L-Phe. To improve the yield of L-Phe during this crystallization step, it is required to increase the concentration of L-Phe in the hydrolyzate. However, in the neutralization process, L-Asp is also precipitated during the step of increasing the pH by adding aqueous alkali. Particularly, at a pH of about 1, L-Asp begins to crystallize, and the ability to stir the crystallization slurry becomes extremely difficult.
Increased concentration of L-Phe results in an increased concentration of L-Asp which exists in an almost equimolar amount to L-Phe. Accordingly, the solids content of the slurry becomes too high at the pH range around 1. In some cases, the entire or a portion of the crystallization slurry may become solidified and cannot be stirred. Accordingly, to improve the recovery of L-Phe, it is required to improve the ability to stir the slurry during the pH adjustment.
Moreover, in the conventional neutralization method, the deterioration of the ability to stir the slurry results in smaller L-Phe crystals. Further, the L-Phe crystals obtained after separation have a high water content. Using L-Phe with a high water content as a raw material for production of .alpha.-APM disadvantageously requires an extremely high load for drying when the L-Phe is recycled in the process described in U.S. Pat. No. 4,173,562. In addition, when L-phenylalanine is converted into its methyl ester for recycling, for example, as in the processes disclosed in U.S. Pat. No. 3,786,039 and Japanese Patent Application Laid-Open No. 92729/1978, the yield of ester may be lowered, because esterification is in equilibrium with hydrolysis.
Thus, there remains a need for an industrial process to recover L-Phe with a low water content in high yield when L-Phe is crystallized and recovered by neutralization from a mineral acid-hydrolyzed reaction solution of a side flow, generated from .alpha.-APM.